Encoder tests sometimes fail on reading input

I've noticed that some of the encoder tests sometimes fail on reading input:

FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_file-2-2] - RuntimeError: Could not open input file: /tmp/pytest-of-root/pytest-0/test_num_channels_to_file_2_2_0/output.mp3 Invalid data found when processing input
FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_file-None-2] - RuntimeError: Could not open input file: /tmp/pytest-of-root/pytest-0/test_num_channels_to_file_None1/output.mp3 Invalid data found when processing input
FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_tensor-2-2] - RuntimeError: Failed to open input buffer: Invalid data found when processing input
FAILED test/test_encoders.py::TestAudioEncoder::test_num_channels[to_tensor-None-2] - RuntimeError: Failed to open input buffer: Invalid data found when processing input

When this happens locally, these tests usually pass after re-running. I also just saw an instance in CI: https://github.com/pytorch/torchcodec/actions/runs/15593694391/job/43919750761#step:15:730

Start in test/test_encoders.py with TestAudioEncoder::test_num_channels and reproduce the intermittent failures locally. Inspect how the to_file and to_tensor cases produce and read output.mp3, then use the linked CI failure for comparison. Done means the affected encoder tests no longer intermittently fail when rerun locally or in CI.